If, during braking, the disc rotor is brought into frictional contact with the pad, component parts of the disc brake such as pad, undergoes self-excited vibrations, thus causing vibrations of the disc rotor. If the frequency of the self-excited vibrations of the component parts, such as pad, coincides with the natural frequency of the disc rotor, the disc rotor is set into resonant vibrations, thus producing "squeal".
For preventing the "squeal", the conventional practice has been to shift the natural frequency of the disc rotor from the frequency of self-excited vibrations of the component parts such as pad. In effect, it has been practiced to provide an opening or a groove on the braking surface of the disc rotor (as disclosed in, for example, JP Utility Model Kokai JP-UM-A-54-108880), to have an annular metallic member on the outer periphery of the disc rotor (as disclosed in, for example, JP Patent Kokai JP-A-56-164236) or to use a larger circumferential width of a disc rotor fin (as disclosed in, for example, JP Patent Kokai JP-A-59-126124). There is also disclosed in JP Patent Kokai JP-A-58-57529 a disc rotor in which, for increasing the vibration attenuation capability of a pressure contact portion of the disc rotor rim and for increasing the strength of the center mounting portion as compared to that of the pressure contact rim portion, the cooling velocity of the molten cast iron at the pressure contact rim portion is set to a different value from that at the center mounting portion so that the graphite at the pressure contact rim portion and that at the center mounting portion of the disc rotor will be flaky graphite and fine graphite, respectively.